1. Technical Field of the Invention
The present invention relates generally to an improved structure of a gas sensor element which is quickly activatable without sacrificing output characteristics thereof which may be built in a gas sensor employed in combustion control for automotive internal combustion engines.
2. Background Art
There are known exhaust emission feedback control systems which have an air-fuel ratio (A/F) sensor installed in an exhaust pipe of an automotive engine to measure the concentration of oxygen (O2) contained in exhaust emissions and determine an air-fuel ratio in the engine as a function of the measured value for controlling combustion of the engine.
Particularly, when a three-way catalyst is used to convert air pollutants contained in exhaust emissions of automotive engines into harmless products, it is essential to control combustion of the engine so as to keep an air-fuel ratio of a mixture supplied to each combustion chamber of the engine within a limited range.
Such air-fuel ratio control typically employs an A/F sensor which is equipped with a laminated gas sensor element having a measurement gas chamber into which exhaust gasses are introduced through a diffusion resistor from outside the gas sensor element, a measurement gas electrode which is affixed to an oxygen ion-conductive solid electrolyte plate and exposed to the measurement gas chamber, and a reference gas electrode which is affixed to the solid electrolyte plate and exposed to a reference gas. The solid electrolyte plate, the measurement gas electrode, and the reference gas electrode constitute an electrochemical cell which works as a sensor cell to measure the concentration of oxygen within the measurement gas chamber.
For example, U.S. Pat. No. 6,340,419 B2, assigned to the same assignee as that of this application teaches an A/F sensor of the type as described above, the disclosure of which is totally incorporated therein by reference.
In recent years, there has been an increasing need for a gas sensor element of the A/F sensors used in the exhaust emission feedback control systems to have the ability to be activated quickly and high measurement accuracy.
Particularly, the quick activation of the A/F sensor element is one of objects of the exhaust gas regulations and is essential to reduce a large amount of hydrocarbon (HC) emitted greatly at cold start-up of engines.
The quick activation of the A/F sensor element requires a reduced heat capacity of the A/F sensor itself, that is, a reduction in size of the A/F sensor element and an increase in quantity of thermal energy produced to heat the A/F sensor element for activation thereof.
A rapid rise in temperature of the A/F sensor element for activation thereof, however, may lead to various concerns about output characteristics of the A/F sensor element.
The A/F sensor element of the above type is typically equipped with a diffusion resistor and designed to produce as a sensor output a limiting current in a one-cell type or a pump current in a two-cell type. Specifically, exhaust gasses are introduced through the diffusion resistor into a measurement gas chamber formed in a body of the A/F sensor element and then interact with a measurement gas electrode installed in the measurement gas chamber to produce the sensor output.
It is essential to heat the A/F sensor element up to a given activation temperature for producing the sensor output correctly. In the following discussion, a period of time until the A/F sensor element is activated sufficiently will also be referred to as an active transition period below.
During the active transition period, the gas staying in the measurement gas chamber of the A/F sensor element is heated rapidly so that it expands, but however, expelling of the gas from the measurement gas chamber is disturbed greatly by the diffusion resistor. This will result in a difficulty in introducing exhaust gasses into the measurement gas chamber when the A/F sensor element has reached the activation temperature, which leads to an error in determining an A/F ratio in the engine using the sensor output.
FIG. 14 demonstrates a time-sequential change in output of the A/F sensor element which was measured in the atmosphere where a A/F ratio is 18 (i.e., N2/O2=4%).
A heater installed in the A/F sensor element was turned on to heat a body of the A/F sensor at a time 0. When the temperature of the A/F sensor element was low, it produced no sensor output. Upon a rise in temperature of the A/F sensor element, it started to produce the sensor output. After the A/F sensor element was warmed up completely, the sensor output should be kept constant, as indicated by a broken line, but actually varied, as indicated by a solid line.
In a case where the A/F sensor element is installed in an exhaust pipe of the engine, when the engine is at rest, that is, when the A/F sensor element is in an inactive state at an ambient temperature, the air entering from outside the exhaust pipe would exist around the A/F sensor element, so that the gas staying in the measurement gas chamber has substantially the same concentration of oxygen as that of the air. This often causes an output of the A/F sensor element such as the one, as indicated in FIG. 14, to indicate, in error, an A/F ratio leaner than that in the engine.
When the A/F sensor element has been activated, and the rise in temperature of the A/F sensor element has been stopped, the gas staying in the measurement gas chamber stops expanding and is replaced with gas flowing outside the A/F sensor element by a pumping operation of the A/F sensor element, so that the sensor output is kept constant, as illustrated in FIG. 14.
Specifically, the output error of the A/F sensor element will result in a delay in producing a correct sensor output, which is objectionable to the quick activation of the A/F sensor element.
There has been proposed the following measures to alleviate the above problem.
Rapid expelling of the gas staying in the measurement gas chamber is achieved by decreasing the degree of diffusion resistance of the diffusion resistor. This, however, results in a difficulty in producing a limiting current as well as a decrease in the output error of the A/F sensor element, thus decreasing the measurement accuracy of the A/F sensor element.
Usually, an exhaust gas emitted from automotive engines is subjected to a great change during operation of the engine. When the pressure of gas to be measured by the A/F sensor element varies, the decreased diffusion resistance may also result in instability (i.e., pulsation) of the sensor output arising from the variation in the gas.
The output error is eliminated by decreasing a rate at which the A/F sensor element is heated up so as to permit the gas to be discharged from the measurement gas chamber completely before the temperature of the A/F sensor element reaches a activation temperature thereof. This, however, results in a difficulty in activating the A/F sensor element quickly.
The above problems are also encountered by another type of gas sensors which are designed to introduce a gas to be measured into the measurement gas chamber through the diffusion resistor and required both to be activated quickly and to have high measurement accuracy.